The present invention relates to a system and corresponding method for controlling mobile station (MS) uplink power as a function of uplink interference in a particular cell defined by a base station(s).
In cellular communications systems, service areas are formed by small zones known as xe2x80x9ccells.xe2x80x9d Each cell is defined by a particular base station (BS), so that it is possible to achieve effects of a transmission power reduction with an increase in subscriber capacity. One type of radio access scheme which has become increasingly popular over the last several years is known as code division multiple access (CDMA). In a CDMA system, since the same frequency band is shared by multiple users, signals of other users become interference signals which may degrade the communication quality of a particular user. When a base station (BS) communicates with near and remote mobile stations (MS) at the same time, it receives the transmitted signal from the near mobile station at a high level, whereas from the remote mobile station at a much lower level. Thus, communications between the base station and the remote MS present a problem in that the channel quality is sharply degraded by interference from the near MS. This is typically referred to as the near-far problem.
One technique which has been used for solving the near-far problem is controlling transmission power such that the received power of a receiving station, or the SIR (signal to interference plus noise power ratio) thereof, is kept fixed regardless of the location of a MS. This provides more consistent channel quality across a given service area. In other words, in CDMA systems the output power of mobile stations is often controlled, with the goal of transmitting at a power such that the received signal quality at the BS is just sufficient for the desired quality of reception. Such control depends upon the conditions of the signal at issue and upon interference (i.e., interfering signals).
In this regard, a closed loop transmission power control system for CDMA is known which employs transmission power control bits. In this system, when a MS within a cell communicates with the base station (BS) in that cell, it measures the received SIR of the desired wave from the BS, and determines the transmission power control bits for controlling the transmission power of the BS in accordance with the measured results. Subsequently, the MS inserts the transmission power control bits into its transmitted signal and transmits them to the BS. Receiving the signal transmitted from the MS, the BS extracts the transmission power control bits therefrom and determines the BS transmission power in accordance with the command of the transmission power control bits.
Still further, the BS measures the received SIR of the wave received from the MS and determines the transmission power control bits for controlling the transmission power (i.e., uplink power) of the MS on the basis of these measurement results. Then, the BS inserts the transmission power control (PC) bits into the transmitted signal, which is transmitted to the MS. Receiving the signal from the BS, the MS extracts the transmission power control (PC) bits and determines the transmission power (i.e., uplink power) in accordance with the instructions of the transmission power control (PC) bits. The closed loop thus formed between the MS and the BS enables each of the MS and BS to control its transmission power.
Unfortunately, the amount of interference in a particular cell increases along with an increase in the number of mobile stations being used in that cell. When a signal is added in the cell, all mobiles stations will typically want to increase their uplink or transmit power until the quality of the signal received at the BS is again sufficient. Such a system can operate in a satisfactory manner so long as the sum of all signals in the cell is below a critical or predetermined value. When it goes above or over this value, all users or mobile stations will constantly increase their power; this is sometimes called the xe2x80x9cparty effect.xe2x80x9d When this occurs, one or more signals are typically removed (i.e., certain MS connections are terminated) until most of the users reduce their power to levels such that the sum in the cell is below the critical value. During this time of overload, control is speculative at best.
It is undesirable to react to overload by immediately cutting of certain MS connections. Alternatively, if one attempts to solve the aforesaid overload or party effect problem by increasing the uplink power from each MS, the result is increased interference in the cell at issue as well as in other cells. This is also undesirable. Moreover, when measures are taken to solve the overload problem, it often takes quite a bit of time for the system to again reach normal operation.
It would also be undesirable to set a concrete limit on the UL power which may be utilized by mobile stations once the critical value of UL interference has been reached, for at least the following reasons. Assume that mobile stations A, B, and C are operating in a particular cell. The base station detects UL interference having exceeded the threshold or critical value. Assume that the base station in turn instructs these three mobile stations that they cannot increase their UL power. Problems occur when one of these mobile stations (e.g., MS A) travels further from the base station and towards the cell border. Because it is further from the BS and closer to the border, it requires additional UL power. However, if a limit has been placed on it, it is not allowed to increase power (even if such an increase would not undesirably increase the UL interference in the cell due to the MSxe2x80x2 distance from the BS). Thus, while placing concrete UL power limits on mobile stations may be able to reduce total UL interference in a particular cell, certain mobile stations in the cell may be undesirably handicapped or accidentally cut off as a result of the same (e.g., when certain mobile stations travel away from the BS and toward the cell border).
In view of the above, it will be apparent to those skilled in the art that there exists a need for addressing UL interference problems in cells of cellular communication networks. Moreover, there exists a need for addressing such problems without having to cut off or undesirably handicap certain MS units which happen to be moving toward the cell border.
In response to calculating or detecting that the uplink (UL) interference has exceeded a predetermined threshold or value in a cell which utilizes closed loop uplink (UL) power control (PC), a base station (BS) instructs a plurality of the mobile stations (MS) currently operating in the cell to reduce their current UL power by a predetermined amount (e.g., value of dB or percentage). Such instruction may override conventional closed loop PC instruction to the mobiles in certain embodiments. In certain embodiments, the base station instructs all mobile stations currently operating in the cell to do so. In such a manner, the UL power reduction is shared in a fairly equal manner by all mobile stations in the cell and adverse effects of overload can be reduced and/or avoided.
As certain mobile stations in the cell move away from the BS and toward the cell border, they may request additional UL power. The base station permits them to increase their UL power, preferably so long as the total UL interference does not increase as a result of the same at the BS. Thus, MS units which happen to be moving toward the cell border are not unduly handicapped relative to other MS units in the cell when a UL interference threshold has been exceeded.
In short, the power of individual mobile stations is not limited (i.e., a concrete limit or ceiling is not put on the UL power of each cell). It is not the output power by the mobiles which is of interest, but instead the received UL interference at the BS which is determinative of total UL interference. The xe2x80x9cpainxe2x80x9d or burden is thus shared in a fairly uniform manner by all mobile stations in the cell. It is not necessary to immediately terminate connections of certain mobile stations in the cell (although this may be done in certain embodiments along with the above UL power reduction instructions). By limiting the UL interference in the cell, all mobile stations suffer a little, but most if not all still have sufficient quality to continue communicating with the BS.